There have been active developments in compound semiconductor devices in which a GaN layer and an AlGaN layer are formed over a substrate and the GaN layer functions as an electron transport layer. An example of such compound semiconductor devices is a GaN-based high electron mobility transistor (HEMT). The GaN-based HEMTs use a two-dimensional electron gas (2DEG) formed in a high concentration at an AlGaN/GaN heterojunction interface.
GaN has a higher bandgap (3.4 eV) than Si (1.1 eV) or GaAs (1.4 eV). That is, GaN exhibits a high breakdown field strength. GaN also has a high saturated electron velocity. Thus, GaN is a potential material for compound semiconductor devices capable of high-voltage operation and high output. GaN-based HEMTs are expected to serve as high-efficiency switching elements and high-voltage power devices in electric vehicles and the like.
GaN-based HEMTs which utilize a high-concentration 2DEG are usually normally-on transistors. That is, an electric current flows when there is no gate voltage applied. This current flow is because of a large number of electrons that are present in the channel. Meanwhile, GaN-based HEMTs used in high-voltage power devices are designed with the highest priority placed on a normally-off operation for failsafe reasons.
A variety of studies have been carried out on normally-off GaN-based HEMTs.
In a GaN-based HEMT illustrated in FIG. 1A, a semi-insulating SiC substrate 201 supports a buffer layer 202, an i-GaN layer 203, an n-AlGaN layer 204, an n-GaN layer 205, an i-AlN layer 206 and an n-GaN layer 207. Two openings are formed in the n-GaN layer 205, the i-AlN layer 206 and the n-GaN layer 207. A source electrode 209s and a drain electrode 209d are provided in the respective openings. The n-GaN layer 205, the i-AlN layer 206 and the n-GaN layer 207 have another opening in a region between the source electrode 209s and the drain electrode 209d. This opening is formed so as to penetrate to a certain depth in the n-AlGaN layer 204. An Al2O3 layer 208 is formed in this opening and extends over the n-GaN layer 207. A gate electrode 209g is provided over the Al2O3 layer 208.
In the GaN-based HEMT illustrated in FIG. 1A, the opening for the gate electrode 209g penetrates to a certain depth in the n-AlGaN layer 204 which functions as an electron supply layer. As a result, when the gate voltage is off, there is no two-dimensional electron gas immediately under the gate electrode 209g. Thus, normally-off operation is possible. FIG. 1B illustrates a conduction band line up for the GaN-based HEMT illustrated in FIG. 1A. As illustrated, a two-dimensional electron gas of higher concentration is obtainable, and thus a large current can flow. The normally-off GaN-based HEMTs thus achieve a high withstand voltage and supply a large current.
However, the GaN-based HEMTs illustrated in FIG. 1A often have a gate leakage current or a lowered withstand voltage. Further, current collapse often occurs.
The related technologies are described in WO 2007/108055, Toshihiro Ohki, “An over 100 W AlGaN/GaN enhancement-mode HEMT power amplifier with piezoelectric-induced cap structure”, Phys. Status Solidi C 6, No. 6, 1365-1368, 2009, and Masahito Kanamura, “A Normally-Off GaN HEMT with Large Drain Current”, IEEJ Trans. Els, Vol. 130, No. 6, 2010.